We've always assumed it was men's sexy faces, toned bodies, and feminist sensibilities that we women lusted after, but it turns out we might just have the hots for their immune systems. A powerful immune system might not sound as enticing as six pack abs, but in the long run it's probably more important. So it makes a fair amount of sense that we'd have developed a radar for who's body can defend itself, and a new study has found we rely largely on testosterone levels to tell who's healthiest.

Researchers from Abertay University in the U.K. recruited 74 Latvian men in their early 20s. They gave them a Hepatitis B vaccine, which triggers the immune system to create antibodies to fight the virus, and they took blood samples right before and one month after the first dose of the vaccine was given. They measured antibody levels, as well as levels of testosterone and the stress hormone cortisol. The researchers also showed photographs of each man to 94 Latvian women, who were also in their early 20s, and had them rate the pictures on a 10-point scale of attractiveness.

After analyzing the men's immune response and hormone levels in connection with their attractiveness score, researchers concluded that high testosterone correlated with both sexy faces and a strong immune response. It's not surprising, really, that women would find the more testosterone-y (that sounds like a pizza topping) faces more attractive, since that tends to come along with more masculine facial features—and who doesn't love a macho man? (Except all of the people who don't love a macho man.)

But it is interesting that men who had the strongest immune responses were perceived as better looking than those with the weakest. This is the first study to document that a women's attraction to a man correlates with the health of his immune system. Of course, a healthy immune system is attractive in a man for any number of reasons—they produce sturdier children, they don't cough all the time, you don't have to take care of them when they're always sick. But who knew we could sense it with our eyes?

They also found that the connection between testosterone and attractiveness was the strongest in men who had the lowest levels of cortisol. Since cortisol is related to stress—which is known to fuck with your immune system—it's likely that having less of it would result in more robust immune function and looking hotter for the ladies. Must be nature's way of keeping us away from people who are stressed out.

Though the research didn't address this, there has to be some kind of an upper limit, though, on how high testosterone levels can be before men begin to seem extremely unattractive and unhealthy. For instance, body builders who dope themselves up on steroids until they look like they're going to pop are not universally considered the hottest men in all the land.

Anyway, more research needs to be done to make sure this info holds for people who aren't Latvians in their mid-20s, but this does establish a pretty interesting connection between who's hot, who's loaded with testosterone, and who's healthy. It's probably only a matter of time before we're eagerly awaiting the release of People's Sexiest Immune System Alive issue.